1. Field of the Invention
The present invention relates to an exhaust emission control system and method for an internal combustion engine and an engine control unit, for reducing exhaust emissions by collecting particulates from exhaust gases discharged from an internal combustion engine, particularly from a diesel engine, installed on a means of transportation, as a drive source.
2. Description of the Related Art
In general, in a diesel engine (hereinafter referred to as “the engine”) which uses an exhaust emission control system of the above-mentioned type, as the amount of particulate matter deposited on a filter increases, exhaust pressure rises, which results in a lowered engine output and decreased fuel economy. To avoid the above inconveniences, a regeneration operation for regenerating the filter is carried out. This regeneration operation is performed by carrying out post injection for injecting fuel into a combustion chamber after a combustion stroke in addition to fuel required for combustion of the engine. The post injection causes exhaust gases to contain unburned fuel, and the unburned fuel is burned upstream of the filter in an exhaust pipe to raise the temperature of the filter. As a consequence, the particulate matter deposited on the filter is burned, whereby the filter is regenerated.
During the filter regeneration operation, part of the post-injected fuel is deposited on the wall surface and the like part of the combustion chamber and mixed into engine lubricating oil without being discharged from the combustion chamber, which causes oil dilution. If the oil dilution becomes excessive, the engine lubricating oil cannot fully exert the effects of its lubricating function, so that it is impossible to maintain a normal operation of the engine. To solve this problem, a control system for an internal combustion engine, for suppressing oil dilution has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2003-322044.
This control system is for a gasoline engine. In the control system, at the start of the engine, fuel injection timing is set according to the count value of a dilution counter. The count value is calculated as follows: First, a coolant temperature at the start of the engine is stored, and during operation of the engine, the integral value of the amount of intake air is calculated. Then, if the coolant temperature and the integral value of the amount of intake air are not larger than respective predetermined values when the engine stops, the counter value is incremented. This process is executed whenever the engine is operated. Then, when the counter value exceeds a predetermined value, it is judged that excessive oil dilution may have occurred, and at the start of the engine, the fuel injection timing is set to a further advanced value so as to suppress the oil dilution.
However, in a case where the conventional control system is applied to a diesel engine having the aforementioned filter, if post injection timing is set to an advanced value so as to suppress oil dilution, the supplied fuel becomes easy to burn in the combustion chamber, and hence unburned fuel required for filter regeneration cannot be supplied to the exhaust system of the engine, which makes it difficult to fully regenerate the filter. Further, the output torque increases for the same reason. On the other hand, if the post injection is set to such timing as will enable filter regeneration, part of fuel remains in the combustion chamber, which prevents reliable suppression of oil dilution.